The present invention relates to a variable wavelength semiconductor laser which is required mainly for wavelength multiplex optical fiber communication, particularly to a variable wavelength semiconductor laser of multiple-electrode DBR structure wherein distributed Bragg reflectors (DBR) made of a semiconductor are disposed before and after an active region and a phase control region, and a control electrode is provided in each region.
An example of a variable wavelength semiconductor laser having a multiple-electrode DBR structure, specifically a sampled grating DBR laser which uses a sampled grating for the diffraction grating portion, will be described below. FIG. 11 is a schematic sectional view of a device having the constitution of sampled grating DBR 20 laser of the prior art described by V. Jayaraman et al. in IEEE J. Quantum Electronics, vol. 29, No. 6, 1993, pp 1824-1834.
In FIG. 11, reference numeral 1 denotes a forward light reflection region (also referred as forward mirror), 2 denotes a backward light reflection region (also referred as backward mirror), 3 denotes an active region, 4 denotes a phase control region, 5 denotes an n-type electrode, 6 denotes an n-type InP lower cladding layer. 7 denotes a p-type InP upper cladding layer, 8 denotes a p-type InGaAsP contact layer, 9 and 10 denote pitch modulation periods of the forward mirror and the backward mirror, respectively, 11 denotes a laser beam emitted from the forward end face of a laser resonator, 12 denotes an InGaAsP optical waveguide layer, 13 denotes a diffraction grating portion, 14 denotes discontinuity in the diffraction grating, and 15 denotes a p-type electrode.
The forward light reflection region 1 and the backward light reflection region 2 have sampled grating DBR mirrors formed on the InGaAsP optical waveguide layers on the forward end face side and the backward end face side of the laser resonator, respectively. Combined length of one diffraction grating portion 13 and one non-diffracting portion 14 (portion without diffraction grating formed therein) is called the pitch modulation period. In FIG. 11, the pitch modulation period of the forward mirror is indicated by reference numeral 9 and the pitch modulation period of the backward mirror is indicated by reference numeral 10. Each of the forward mirror and the backward mirror has a plurality of pitch modulation periods, while the pitch modulation period of the forward mirror and the pitch modulation period of the backward mirror are set to different values in general.
An optical waveguide of a laser oscillator has an InGaAsP layer sandwiched by the lower cladding layer 6 and the upper cladding layer 7 which have a forbidden band gap greater than that of the InGaAsP layer, while the backward light reflection region 2, the active region 3, a phase control region 4, and the forward light reflection region 1 are formed on the InGaAsP layer.
The active region 3 is constituted from an n-type InGaAsP strained quantum well which has a forbidden band gap smaller, on average, than the forbidden band gap of the InGaAsP optical waveguide layer 12 that constitutes the forward light reflection region 1 and the backward light reflection region 2, and the phase control region 4 comprises an InGaAsP optical waveguide layer having the same composition as that of the forward light reflection region 1 and the backward light reflection region 2.
Now the operation of the sampled grating DBR (SSG-DBR) laser of the prior art shown in FIG. 11 will be described below.
As shown in FIG. 11, the p-type electrodes 15 are formed separately on the active region 3, the forward light reflection region 1, the backward light reflection region 2 and the phase control region 4. When a forward bias voltage is applied between the p-type electrodes 15 which are formed separately on the active region 3 and the n-type electrode 5, current flows into the active region 3 so that spontaneous emission of light ranging over a broad band of wavelengths takes place in the active region 3. The emitted light propagates through the optical waveguide formed in the optical resonator, and light of a particular wavelength is reflected on a forward sampled grating DBR mirror formed in the forward light reflection region 1 and a backward sampled grating DBR mirror formed in the backward light reflection region 2 repetitively and is amplified in the active region, thus achieving laser oscillation.
In the sampled grating DBR laser shown in FIG. 11, supplying a current to the forward light reflection region 1 or the backward light reflection region 2 and the phase control region 4 results in a wavelength to be selected so that, at the single wavelength selected according to the current supplied, laser oscillation occurs.
A process of controlling the oscillation wavelength of the laser shown in FIG. 11 will now be described in detail.
FIG. 13 shows a reflectivity spectrum 16 of the forward mirror 1 and a reflectivity spectrum 17 of the backward mirror 2 in case current is not supplied in the forward and backward light reflection regions 1, 2. FIG. 14 shows a reflectivity spectrum 18 of the backward mirror 2 in case current is supplied only to the backward light reflection region 2 and the reflectivity spectrum 17 of the forward mirror 1 without current supply in comparison.
In FIG. 13 and FIG. 14, wavelength is plotted along the abscissa and power reflectivity is plotted along the ordinate. xcex1 is a wavelength at which the peaks of reflectivity of the forward and backward mirrors coincide in case current is not supplied to the forward and backward light reflecting regions 1, 2, and xcex2 is a wavelength at which the peaks of reflectivity of the forward and backward mirrors coincide in case current is supplied only to the backward light reflecting region 2.
The reflection spectrum of the sampled grating DBR consists of a plurality of sharp peaks of reflectivity which are different in height, although all peaks of reflectivity are shown in the diagram to have equal height since it is the sole object to show the wavelengths in FIG. 13 and FIG. 14.
As described previously, FIG. 13 shows the reflectivity spectrum in the initial state when the forward mirror control current and the backward mirror control current are both zero. In this case, the reflectivity spectra of the forward and the backward mirrors coincide at wavelength xcex1. As a result, power loss of light becomes extremely smaller at the wavelength xcex1 compared to the other wavelengths, which means relatively high gain of light at the wavelength xcex1 and therefore the laser oscillates at the wavelength xcex1.
The oscillation wavelength of the laser can be changed by applying a forward bias voltage to one or both of the forward light reflection region 1 and the backward light reflection region 2 to supply current to the region, equivalently changing the refraction indices of the light reflection regions 1 and 2 by way of free carrier plasma effect.
As shown in FIG. 14, for example, in case current is supplied only to the backward light reflection region 2, the reflectivity spectrum 18 of the backward mirror shifts to a shorter wavelength due to a decrease in the refraction index of the backward mirror, and the wavelength at which the reflectivity spectra of the forward and backward mirrors coincide changes to xcex2. As a result, light of wavelength xcex2 propagates in the laser resonator and is amplified so that laser oscillation eventually occurs. The oscillation wavelength can be changed similarly in case current is supplied only to the forward light reflection region 1 and in case current is supplied to both the forward light reflection region 1 and the backward light reflection region 2. Thus oscillation wavelength of the laser can be changed freely by supplying current to the light reflection regions where the mirrors are formed and controlling the intensity of the current supplied thereby changing the refraction index.
FIG. 12 shows an enlarged portion of a cross section of the sampled grating DBR for showing the direction of current flow when current is supplied to the InGaAsP optical waveguide layer 12, which constitutes the double hetero-structure, such as the forward light reflection region 1 and the backward light reflection region 2, namely when current is caused to flow by applying a forward bias voltage across the p-type electrode 15 of the forward light reflection region 1 or the p-type electrode 15 of the backward light reflection region 2 and the n-type electrode 5. In FIG. 12, reference numeral 19 denotes mirror modulation period xcex90, 20 denotes length xcex9g of the diffraction grating portion and 21 denotes the current supplied. The diagram is simplified by omitting the p-type electrode 15, the n-type electrode 5 and the p-InGaAsP contact layer 8. Direction of current supplied is indicated by the arrows.
When the forward bias is applied, positive holes are injected from the p-type electrode 15 through the p-InP upper cladding layer 7 and electrons are injected from the n-type electrode 5 through the n-InP lower cladding layer 6 into the InGaAsP optical waveguide layer 12. At this time, the supplied current 21 flows into the InGaAsP optical waveguide layer 12 uniformly over the entire region thereof, so as to equivalently decrease the refraction indices of the optical waveguides by way of the free carrier plasma effect. The decrease in the refraction index caused by the current supply causes a blue shift of the reflectivity spectrum 17 of the backward mirror as a whole, i.e., toward shorter wavelengths, so that the wavelength at which the peaks of reflectivity of the forward and backward mirrors coincide changes, thus making it possible to change the oscillation wavelength. Thus laser oscillation wavelength can be freely selected and controlled by equivalently changing the refraction index of the forward light reflection region 1 or the backward light reflection region 2 which constitutes the DBR mirror, in accordance with the current supply. The phase control region 4 is provided for the purpose of making fine adjustment of the phase of laser emission by applying the forward bias and supplying current to the region 4, thereby to stabilize the laser oscillation.
In a semiconductor laser device of the prior art such as that described above, laser oscillation wavelength can be freely selected and controlled by supplying a proper level of current to the forward light reflection region 1 or the backward light reflection region 2 so as to change the wavelength, and controlling the level of current supplied thereby equivalently changing the refraction index of the forward or backward light reflection region.
However, this results in such a state that the electrons and the positive holes injected into the forward light reflection region 1 or the backward light reflection region 2 coexist in the same semiconductor, namely in the same space. Such a state leads to recombination of the electrons and the positive holes, which causes spontaneous emission of light that occurs randomly due to the recombination. As a result, density of carriers in the forward or backward light reflection region varies, in the so-called carrier density fluctuation. The carrier density fluctuation in the light reflection region causes problems of deteriorating laser performance such as broadening of the line spectrum of laser light under wavelength control, and the problems become more conspicuous as the current supplied into the light reflection region increases.
The laser of the prior art also has such a problem that, since a certain amount of current is supplied to the. light reflection region, heat is generated accordingly. The heat generated by the current supply increases the refraction index of the light reflection region equivalently. The change in the refraction index due to the heat generation has a time constant that is fairly larger than that of the change in the refraction index caused by the free carrier plasma effect, and therefore it takes a longer time to control the laser oscillation wavelength and stabilize the oscillation. There is also such a problem that, when current supply to the light reflection region is continued, the oscillation wavelength changes over time due to deterioration of the device.
Thus in the semiconductor laser device of the prior art, recombination of the carriers occurs in the light reflection region when controlling the wavelength, and the recombination of the carriers causes broadening of the line spectrum of laser oscillation and an effective decrease in the optical output of the laser, thus resulting in such problems as channel interference when used in wavelength multiplex communication or coherent communication. The shift in wavelength due to the heat generated by current supply causes a drift in the laser oscillation wavelength particularly in case a large current is supplied to achieve a large change in the wavelength, thus resulting in the undesirably long time being taken to control the wavelength and other significant problems in the high speed operation, stability and other performance required for a light source used in optical communication
The present invention has been made to solve the problems of the semiconductor laser device of the prior art described above. The present invention provides a high performance single-wavelength semiconductor laser device which is capable of stable and high-speed operation with sufficiently small width of oscillation spectrum line and less shift or drift of the oscillation wavelength caused by the heat generation, by forming a current blocking layer in a portion of the light reflection region which controls the wavelength where diffraction grating is not formed, in contact with the light reflection region, thereby to suppress the injection of carriers into the light reflection region and the recombination of electrons and positive holes.
Specifically, the semiconductor laser device of the present invention includes an active region, a forward light reflection region located in front of the active region, a backward light reflection region located behind the active region and a phase control region located in proximity to the active layer, which are provided between the upper cladding layer and the lower cladding layer, wherein the forward light reflection region and the backward light reflection region are constituted by disposing diffraction grating portions and non-diffracting portions alternately, while the laser oscillates at a wavelength which corresponds to the current flowing in the diffraction grating portion, the semiconductor laser device being characterized in that a current blocking layer is formed on the non-diffracting portion of at least one of the forward light reflection region and the backward light reflection region, for the purpose of blocking the current from flowing into the non-diffracting portion.
In the semiconductor laser device of the present invention having such a constitution as described above, since the current blocking layer is provided in the light reflection region which controls the wavelength so as to prevent the current from flowing into the non-diffracting portion other than the diffraction grating portion where the diffraction grating is formed, thus electrons and positive holes are separated from each other in space in the optical reflection region other than the diffraction grating portion so that recombination of carriers do not occur when controlling the laser oscillation wavelength. This makes it possible to greatly reduce the occurrence of spontaneous emission of light and the variation in the density of carriers which accompany the carrier recombination, compared to the prior art, thus achieving laser oscillation having a narrow oscillation spectrum line. Also because the heat generated by the current supplied can be reduced, shift in the oscillation wavelength due to the heat can also be reduced, thus greatly improving the basic characteristics of the laser. Since the diffraction grating portion is typically designed to have smaller width than the non-diffracting portion, the laser characteristics can be greatly improved by the present invention. Even more remarkable effect can be achieved by designing the semiconductor laser device to have narrower diffraction grating portion.
Thus the present invention can provide a high performance single-wavelength semiconductor laser device which allows the oscillation wavelength of laser to be freely controlled and is capable of stable and high-speed operation with high reliability.
Therefore, use of the semiconductor laser device of the present invention in the wavelength multiplex communication or the coherent communication makes it possible to suppress channel interference and wavelength drift which have been posing problems in the wavelength multiplex communication or the coherent communication, and mitigate the problem of decreasing optical output of the laser.
In the semiconductor laser device of the present invention, it is preferable to form the current blocking layer in the non-diffracting portions of both the forward light reflection region and the backward light reflection region.
In the semiconductor laser device of the present invention, the current blocking layer can be comprised of a high-resistance layer which has a resistance higher than that of the non-diffracting portion.
Further in the semiconductor laser device of the present invention, in case the upper cladding layer has a first conductivity type and the lower cladding layer has a second conductivity type, the current blocking layer can also be constituted from a first semiconductor layer of the first conductivity type formed on the non-diffracting portion and a second semiconductor layer of the second conductivity type formed on the first semiconductor layer.
Also in the semiconductor laser device of the present invention, such a constitution may also be employed as the upper cladding layer is made of p-type InP, the lower cladding layer is made of n-type InP, the first semiconductor layer is made of p-type InP and the second semiconductor layer is made of n-type InP.
Also in the semiconductor laser device of the present invention, the current blocking layer may also be made of a semi-insulating semiconductor.
Furthermore, such a constitution may also be employed as the upper cladding layer is made of p-type InP, the lower cladding layer is made of n-type InP and the current blocking layer is made of semi-insulating InP.
Moreover in the semiconductor laser device of the present invention, such a constitution may also be employed that a heater is provided so as to change the temperature of one of the forward light reflection region and the backward light reflection region, and the current blocking layer is formed in the non-diffracting portion of the other light reflection region so as to -block the current from flowing in the non-diffracting portion, in which case the wavelength can be changed more effectively.